Numerous inventions address active and passive refrigerated or heated thermal enclosures. Active heat pumps, based on refrigeration systems such as vapor compression, adsorption, or thermoelectric devices provide tight-tolerance temperature control when used in conjunction with a closed-loop control system. These systems, however, require a large amount of electrical power and hence do not serve well for long duration shipping containers that may require from two to five days of temperature control under severe ambient environments. Passive systems using ice packs or dry ice may provide sufficient refrigeration for two to five days of shipping, but do not provide active temperature control. Active temperature control is very important when transporting temperature sensitive products such as vaccines or other pharmaceutical products.
One major problem with current shipping container enclosures is that the refrigerants, which might be designed to keep a product in a temperature range of two to eight degrees Celsius ("C."), may freeze the products. Even if the refrigerant has its phase change at zero degrees C., such as with water, the ice packs are typically frozen in a much colder environment to reduce the time it takes to freeze them. Consequently, when the packs are removed from a common industrial freezer that typically operates at -15 to -25.degree. C., they may be as cold as the operating temperature maintained by the freezer. Depending on the insulative value of the enclosure and the amount of ice packs added, it could take many hours before the refrigerant ice packs warm up to the phase change temperature. During this time it is common for the temperature inside the enclosure to drop below freezing, thereby destroying the efficacy of the products. Damaged pharmaceutical products can be hazardous in two ways: (1) they can lose their efficacy and not perform their intended function; or (2) they can themselves become toxic.
A second common problem is that passive shipping containers with refrigerant packs are often inadequate to withstand high external temperatures. In any given packaging configuration of a passive enclosure with refrigerants, there is a fixed amount of thermal resistance between the payload (e.g. drug product) and the refrigerants. In high heat load environments, this fixed thermal resistance is often too high for the refrigerant to keep the contents of the entire enclosure below the maximum temperature to which the products are validated (typically 8.degree. C. for a drug product). Consequently the fixed resistance nature of the "ice-packs in a foam box" approach is inadequate to keep the products below their maximum temperature control point. Similarly, a common problem is that passive shipping containers with refrigerant packs are often inadequate to keep the products from freezing when the enclosure is in a cold environment. In this situation, the fixed thermal resistance between the payload and refrigerants is too low, and consequently the refrigerant continues to cool the product even though the outside temperature is very low. Hence the inability for a passive shipping container with refrigerant packs to vary the thermal resistance between the refrigerant and payload is a major drawback.
Actively controlled thermal enclosures can sometimes overcome the shortcomings associated with the fixed-thermal-resistance-packaging approach described above. However, these systems have their own unique set of problems. In a shipping container application, the large amount of energy required to operate an actively powered heat pump is problematic. Even with the use of high R-value vacuum insulation panels, a large battery is required to maintain a tight-temperature tolerance over several days. Also, actively powered refrigeration systems require that the heat removed from the enclosure be rejected to the external environment. This is a problem if shipping containers are shrink-wrapped for shipment or if numerous containers are placed in a closed volume such as the back of a panel truck. If proper heat rejection is not achieved, the system will loose its ability to regulate the temperature inside the enclosure, resulting in product damage.
Another problem associated with actively powered thermal enclosures is that a heat pump is limited by the temperature difference it can maintain. As the ambient temperature increases, the temperature difference across the heat pump increases accordingly. At the same time, more thermal energy, or heat leak, enters the enclosure. That heat must be removed and rejected by the heat pump. That increases the temperature of the heat rejection device and further increases the temperature difference across the heat pump. Furthermore, if the heat rejection environment is affected by the increase in energy discharged into it, it will rise in temperature, further increasing the temperature difference across the heat pump. Not only can this scenario consume large amounts of battery power, it will ultimately lead to the system's inability to maintain the enclosure's internal temperature below its set point.
In a more stationary thermal enclosure application, such as with home refrigeration, in which electrical power is more readily available and the external temperature does not vary greatly, internal temperatures are more easily maintained. However, in this application, power consumption is high and refrigerators and freezers are reported to be the highest energy consuming appliances in a home. The power consumption is affected by the insulative value of the enclosure, the temperature difference between the outside and inside of the enclosure, the efficiency of the heat pump system, and, in some cases, the reverse heat leak through the heat pump when it is not actively being used, as is the case with a thermoelectric heat pump. A means to thermally disconnect the heat pump from the system when not in use would greatly reduce the heat leak into the system and reduce power consumption. Energy consumption would be further reduced if a thermal energy storage system were used, so that the heat pump might only operate for the time required to freeze a phase change refrigerant, such as water. This would allow the heat pump to be operated when energy cost were lower, such as during night hours. Energy would also be saved by operating the system at night because the heat rejected from a refrigerator in a home must be rejected by the home's air conditioning system, which will run more efficiently at night when external temperatures are typically cooler. In applications such as vending machines, in which power is often available, it would be even more important to be able to operate the heat pump only during evening hours when external temperatures are much lower and the heat pumps run more efficiently, consuming less power. For vending machines in remote areas, or for refrigerators in areas where no power is available, the thermal storage system would be very valuable because the system could be recharged from intermittent energy sources such as solar panels, gas powered generators, or even generators driven by campfires.